1. Field of the Invention
The present invention relates to an apparatus of multi-beam type having a plurality of charged beam optical systems and a method of charged beam lithography using such a charged beam lithography apparatus.
2. Discussion of the Background
An electron beam lithography apparatus has conventionally been used widely for developing advanced devices. The conventional electron beam lithography apparatus is such that an electron beam is electro-magnetically or electrostatically deflected to draw a desired figure on a workpiece. In the case where the electron beam is simply deflected, however, a deflection distortion occurs as shown in FIG. 1A. It is, therefore, necessary to adjust the drawing position before the drawing operation as shown in FIG. 1B.
The drawing position is adjusted generally in the manner described below.
A stage is moved by the distance of beam deflection and a voltage or a current applied to a deflection electrode or a coil is determined in such a manner that a mark formed on the stage is detectable. In the process, some marks used for beam adjustment take advantage of the difference in the atomic number between each mark and the substrate or in the electron reflection rate due to the unevenness. The size of the beam deflection area is normally several hundred .mu.m to several mm. Before drawing in a range beyond the deflection area, the stage is moved into the beam deflection area.
In FIGS. 1A and 1B, reference numeral 201 designates a deflection area having a distortion, and reference numeral 202 a deflection area of which the distortion is corrected.
In fabricating a semiconductor device, on the other hand, it is a common practice to the base pattern formed on the same substrate is subjected to overlapped exposures. In such a case, the positions of the marks formed on the substrate are required to be detected. Some position-detecting marks take advantage of the difference in the electron reflection rate due to the unevenness of the marks or due to the difference in atomic number between each mark and the substrate. These marks are scanned by an electron beam and the intensity of the reflected electron signals is measured thereby to detect the mark positions.
A technical development is under way for a variety of electron beam lithography apparatuses aimed at a high throughput. Among them, an apparatus of multi-beam type having a plurality of electron beam optical systems is most promising. An electron beam lithography apparatus of multi-beam type, in which a plurality of beams, not a single beam, are used for drawing, is expected to exhibit a remarkably improved drawing throughput.
The apparatus of multi-beam type requires beam adjustment in each optical system. Sequential adjustment of each beam consumes a very long time. For this reason, in the above-mentioned adjustment of the drawing position or the overlapped exposure, the marks are required to be detected simultaneously for a plurality of beams. The simultaneous mark detection using a plurality of beams, however, poses the problem that an accurate positioning is adversely affected by the electrons reflected from other marks.
Specifically, a given electron optical system, as shown in FIG. 2, includes an electron gun 1, beam blanking systems 3, 4, beam deflection systems 5a, 5b, beam adjusting lens systems 2a, 2b, 2c and a reflected electron detector 6. FIGS. 3A and 3b show a general view of a beam-adjusting mark. FIG. 3A is a plan view, and FIG. 3b is a sectional view. In detecting a mark position, as shown in FIG. 4, a mark 10 is formed in a drawing area 12 corresponding to each electron beam optical system 13. A plurality of marks 10 are scanned by a plurality of beams at the same time, and each mark position is detected from the beam scanning position and a corresponding reflected electron signal. In the conventional electron beam optical system, as shown in FIG. 5, the simultaneous mark detection using a plurality of electron beams 7 makes it difficult to detect individual mark positions by reason of the fact that the reflected electrons 9 from other marks intrude upon the detector 6 sideway together with the reflected electrons 8 from the intended mark.
In a protective measure proposed, the timing of position detection is differentiated for different electron beam optical systems, or the positioning operation is performed not for each chip but for a plurality of chips at a time. These methods, however, pose the problem that the apparatus configuration is complicated and that the mark detection consumes a longer time than when the marks are detected all the electron beam optical systems at the same time.
The multi-beam type of the electron beam lithography apparatus, in which exposure is not collective unlike in the optical stepper or the X-ray exposure, requires beam adjustment for each lens barrel. In the conventional electron beam lithography apparatus having only one electron beam source, only one mark is used for drawing position adjustment. In the multi-beam lithography apparatus requiring many beam adjustments, however, a long time is consumed for beam adjustment when a mark is shared. With the multi-beam lithography apparatus, therefore, a method is required to be employed, in which a reference mark associated with each electron beam optical system is provided and the beam is adjusted for each lens barrel.
The above-mentioned method of adjusting the beam for each lens barrel according to a reference mark corresponding to each electron beam optical system, however, gives rise to the problem described below. Specifically, the chip size is varied with device types, and therefore it is not always the case that each chip corresponds to an electron beam source. As shown in FIG. 6, in the case of drawing a pattern 213 larger than the pitch at which the electron beam optical systems 211 are arranged (more specifically, a drawing area 212 for a given electron beam optical system), the single pattern 213 is drawn using a plurality of electron beam optical systems 211. In the case where electron beam optical systems are arranged at pitches of 10 mm, for example, four or more electron beam optical systems are required for drawing if the chip size is more than 20 mm square. In the case where a workpiece with each side thereof longer than 150 mm such as a photomask is drawn, on the other hand, even more electron beams are used for drawing.
There is no problem encountered in the case where the position of the reference marks 214 of the electron beam optical systems are arranged ideally as shown in FIG. 7A. Actually, however, as shown in FIG. 7B, the arrangement of reference marks should be irregular, so that a pattern drawn on the basis of these reference marks is distorted as shown in FIG. 8. In the case where a given chip is drawn by a plurality of electron beam sources, as shown in FIG. 9, the problem is a deteriorated connection accuracy in the boundaries of the drawing areas of adjacent electron beam optical systems. In FIG. 8, reference numeral 215 designates a drawing area, reference numeral 216 designates a drawing area boundary, and reference numeral 217 designates a drawing pattern.
From the viewpoint of drawing accuracy, the connection accuracy of the defection areas is important. No matter how small the minimum size of a beam, a figure conforming with the design data cannot be drawn on the workpiece if the connection accuracy is low.
As described above, the lithography apparatus of multi-beam type, which uses no collective exposure unlike in the stepper, requires beam adjustment for each lens barrel. Each electron beam source of the multi-beam lithography apparatus has an independent electron beam optical system. Even when the beam is adjusted for each optical system, therefore, the connection accuracy is deteriorated between different electron beams. In addition, the chip size is varied with device types, and it is not always rational to assure correspondence of one chip to one electron beam source. One chip is naturally drawn by a plurality of electron beam sources. The problem posed in the process is a deteriorated connection accuracy in the beam deflection boundary area due to different beam characteristics between different electron beam sources.
In this way, in the conventional electron beam lithography apparatus of multi-beam type, a complicated apparatus configuration is required for mark position detection and a very long time is consumed. Further, when drawing a pattern requiring a drawing area larger than the pitch at which the electron beam optical systems are arranged, the drawing pattern is distorted or the connection accuracy in the drawing areas of adjacent electron beams is deteriorated.
These problems are not confined to the electron beam lithography apparatus, but the same can be said of an ion beam lithography apparatus using an ion beam as a charged beam.